A few selected transitional fossils

Introduction: All species undergo gradual change over time, but in the fossil record we find evidence of some changes that are particularly striking. This website is dedicated to some of these so-called transitional fossils.

Warning 2: When a fossil is called "transitional" between two types of animal, that means it shows some of the traits of both, but it does not mean it links those animals by direct descent. Evolution is a branching process - by which we mean that species often split in two. Therefore:

"Because evolution is a branching process that produces a complex bush pattern of related species rather than a linear process that produces a ladder-like progression, and because of the incompleteness of the fossil record, it is unlikely that any particular form represented in the fossil record is a direct ancestor of any other."

In short, transitional fossils are best thought of as being close relatives of the species which actually link two groups. They may have lived at the same time as those actual links, or they may not have (this confuses many people). As long as these problems are borne in mind, transitional fossils give a rough indication of what evolutionary changes were occurring. But don't be misled into thinking that fossils are the only evidence for evolution. They're not even the strongest evidence for evolution.

Apes - humans

Most ape-like at the top (though in a technical sense, humans are still considered apes). Images and diagrams of the fossils here.

Sometimes called "the only transition which matters", this mustn't be thought of as a transition from chimps to humans, but rather, as a transition from the-common-ancestor-of-chimps-and-humans to humans. Chimps themselves have had time to evolve and change since we parted ways, and so "the ancestor we last shared probably differed substantially from any extant African ape" (White et al, 2009).

Ardipithecus ramidus had a brain the size of a chimp's, but probably walked upright on the ground, while still able to go on all fours in the trees, where it would find its opposable big toe useful (Gibbons, 2009).

Australopithecus afarensis was a more advanced walker, with nongrasping feet (White et al, 2009), but it still had the brain size of a chimpanzee (Dawkins, 2009). Probably not a direct ancestor of modern humans (Rak et al, 2007).

Fish - tetrapods

Update: This group of fossils were thought to be roughly contemporary with the transition onto land. However, recently tracks of a four-footed animal were discovered in marine sediments firmly dated at 397 million years old (Niedzwiedzki et al, 2010). If that animal was a genuine tetrapod, then creatures like Tiktaalik may have been "late-surviving relics" exhibiting transitional features that actually evolved somewhat earlier.

In short, these are not the actual ancestors of modern land animals; but they are related to the actual ancestors, and so they do show us the sort of creature that evolved during the great move onto land.

Though still a water-dweller, Tiktaalik had fins that were halfway towards being feet, and ears capable of hearing in air or water (Prothero, 2007). It was capable of crawling around in very shallow water, and it had a neck, unlike fish but like tetrapods (Coyne, 2009).

Possessing four definite legs, Acanthostega was presumably capable of movement over land (Coyne, 2009), though the legs were still better suited for crawling along the bottom of the water (Prothero, 2007). Its tail was still adapted for propulsion through water, and it still had gills (Ridley, 2004).

Slightly more like a land animal, Ichthyostega had powerful shoulders implying it did indeed use its legs to move over land, at least sometimes (Clack, 2005). Even now, the skull still closely resembled that of Eusthenopteron (Futuyma, 2005).

The foot of Pederpes "has characteristics that distinguish it from the paddle-like feet of the Devonian forms [i.e. the above animals] and resembles the feet of later, more terrestrially adapted Carboniferous forms" (Clack, 2002).

These creatures were related to the lungfish of their time, and almost certainly all had lungs themselves.

It would be a mistake to think that the first tetrapods moving on land needed limbs capable of bearing their full weight; legs sprawled to the side would be enough to move about with. One thing the above fossils seem to show is that legs first evolved for crawling over the bottom of the water; only later did their use on land become paramount.

Dinosaurs - birds

Most dinosaur-like at the top. Images and diagrams of the fossils here.

The famous Archaeopteryx had feathers and was probably capable of at least gliding, but it also had dinosaur-like teeth, claws, and a long bony tail. Its skeleton was "almost identical to that of some theropod dinosaurs" (Coyne, 2009). Precisely how closely related it is to the main line of bird evolution remains the subject of controversy (Xu et al, 2011).

Confuciusornis had a bird-like tail and a pygostyle, which is a feature of modern birds. It retained dinosaur-like claws (Prothero, 2007). It had strong shoulder bones, but was probably not capable of true flapping flight (Senter, 2006). It may have glided. It is the earliest known bird with a toothless beak, but other lineages continued to have teeth for a long time.

Sinornis "still had teeth, an unfused tarsometatarsus, and an unfused pelvis" (Prothero, 2007) but resembled modern birds in other ways, with reduced vertebrae, a flexible wishbone, a shoulder joint adapted for flying, and hand bones fused into a carpometacarpus (Prothero, 2007).

A strong flyer, Ichthyornis was very nearly a modern bird (Prothero, 2007), and yet it still had teeth.

As birds evolved from dinosaurs, and required feathers to fly, the existance of non-flying, feathered dinosaurs is a prediction of evolution. Happily, we have now discovered a significant number of such dinosaurs, one of which, Mei long, was even found curled-up in a remarkably bird-like sleeping position (picture here).

The first feathered dinosaurs found were more recent than Archaeopteryx — feathered dinosaurs didn't die out as soon as birds evolved — but we now have Anchiornis, which has shown that feathered dinosaurs did indeed exist before Archaeopteryx.

One will occasionally encounter vague suggestions that birds evolved from (non-dinosaur) reptiles. John Ruben and colleagues are the main supporters of this idea, but it remains very much a minority view. Michael Benton (who literally wrote the book on vertebrate paleontology) considers the controversy artificially extended by the popular media's obsession with giving both sides "equal time", even though the dinosaur-bird view is supported by the vast majority of working palaeontologists (Benton, 2005). The University of California Museum of Paleontology has a reasonable overview of the dinosaur-bird relationship.

Synapsids - mammals

Most synapsid-like at the top. Images and diagrams of the fossils here.

Strictly speaking, the group that gave rise to mammals were not true reptiles (though they were closely related). Therefore, there is no transition from reptiles to mammals, but rather from synapsids to mammals. However, the terms mammal-like reptiles and reptile-like mammals are still sometimes used for these transitional fossils.

More mammal-like, especially in how it held its limbs: closer to its body like modern mammals, rather than sprawled to the side like Dimetrodon (Prothero, 2007). It still had a great many "primitive" features, such as ribs in the lumbar area (Prothero, 2007).

Had the beginning of a secondary palate in its skull (Prothero, 2007); in modern mammals, this allows eating and breathing at the same time, and is a sign of a more active lifestyle (Ridley, 2004). Its more advanced skull also allowed it to chew its food; and indeed it had premolars and molars with which to do so (Prothero, 2007). The skeleton was not yet fully mammal-like, but it had lost those lumbar ribs.

Probainognathus still possessed a reptile-like jaw articulation (Macdonald et al 2009) but also had "the initiation of the articulation which was later to become the more highly developed glenoid-condyle articulation of the mammal" (Romer, 1969). It had a well developed zygomatic arch (Macdonald et al 2009). However, its braincase was very unlike that of modern mammals (Romer, 1969).

Land mammals - whales and dolphins

Although only a cousin species of the ancestor of whales, Indohyus had bones denser than normal mammals, indicating it was partially aquatic: heavy bones are good ballast (Thewissen et al, 2009). Its ears shared a feature with modern whales: a thickened wall of bone which assists in underwater hearing; non-cetaceans don't have this (Thewissen et al, 2009).

Perhaps the actual ancestor, Pakicetus was probably semi-aquatic; like Indohyus, it had dense bones for ballast (Thewissen et al, 2009). Its body was "wolf-like" but the skull had eye sockets adapted for looking upwards, presumably at objects floating above it (Thewissen et al, 2009). Although initially known from just a skull, many more bones were found later (Thewissen et al, 2001).

With a streamlined, elongated skull and reduced limbs, Ambulocetus probably spent most of its time in shallow water. Its reduced limbs meant it could only waddle on land (Coyne, 2009). It resembled a crocodile in some ways.

The nostrils of Rodhocetus have started to move backwards (towards the blowhole position) and the skeleton indicates a much stronger swimmer (Coyne, 2009). On land it would struggle, moving "somewhat like a modern eared seal or sea lion" (Gingerich et al, 2001). Its teeth were simpler than its predecessors (Futuyma, 2005), a trend that continued to the present.

Seems similar to Rodhocetus. One fossil was found with what appeared to be a foetus, in a position indicating head-first birth (Gingerich et al, 2009) unlike modern whales. However this is disputed; the "foetus" might just be a partially digested meal (Thewissen and McLellan, 2009).

The blowhole in Aetiocetus is about halfway to its position in modern whales on top of the head. Aetiocetus also represents the transition from toothed whales to the filter-feeding baleen whales, being similar to baleen whales in most respects, but possessing teeth (Van Valen, 1968).

Whales evolved relatively quickly. As Coyne (2009) explains, "adapting to life at sea did not require the evolution of any brand-new features - only modifications of old ones". Thewissen et al (2009) give a good overview of whale evolution, which is freely available.

Beware: There's a commonly reproduced image from a popular science book (Zimmer, 1998) that depicts two dolphin-like creatures, Takracetus and Gaviacetus (always misspelled Gaviocetus) as having short hind limbs. However, the first fossils found were basically skulls plus a few vertebrae (Gingerich et al, 1995), and I've been unable to find any discussion of more complete fossils. Zimmer's book is explicit that some of the depictions are based on very incomplete fossils, so I don't regard that image as authoritative. Happily, we have much better fossils for the species I mention in my list.

Protohorses - horses

Body mass of horse species over time; click for larger; after MacFadden (1986)

With horse evolution, it's particularly important to bear the warning at the top in mind: we have many fossils, and the known family tree is very bushy, not a straight line. At one time, 13 different genera of horses existed simultaneously (Raven et al, 2008). Regardless, there are definite trends over time towards larger body size; larger, ridged teeth suitable for grazing; longer limbs; and reduction of side toes (Raven et al, 2008). These trends were not absolute, however.

The skeleton of Parahippus was more adapted to long-distance running, for escaping predators in an open environment (Evans, 1992). About this time, grasslands were becoming common in North America, where horses evolved (Raven et al, 2008). They would later die out in America (Dawkins, 2009).

With bigger teeth, Merychippus was more adapted to the grazing lifestyle of modern horses. Earlier species were likely browsers that ate leaves, but Merychippus could also eat grass (Raven et al, 2008).

Miscellaneous

Aardonyx, a proto-sauropod dinosaur that, though bipedal, could probably also walk on all fours (Yates et al, 2009). Contrary to what you might expect, in this case bipeds evolved to become quadrupeds.

Amphistium, an early flatfish, with eyes intermediate in position between an ordinary fish and a modern flatfish (Friedman, 2008).

Claudiosaurus, an early relative of marine reptiles like plesiosaurs, but the limbs are not very specialised for swimming (Prothero, 2007).

Darwinopterus, a pterosaur, has the advanced skull and neck of the Pterodactyloidea group, but other traits (e.g. its long tail) are like the primitive Rhamphorhynchoid group (Lu et al, 2009).

Enaliarctos, an early seal, but with more primitive skull and feet (Prothero, 2007).

There was a section on ichthyosaurs (a group now extinct) on this site, but I wasn't entirely satisfied with it. We don't yet have anything as good as Tiktaalik or Ambulocetus for ichthyosaurs. Still, fossils like Mixosaurus can be considered transitional between early eel-shaped ichthyosaurs, and later dolphin-shaped species.

Other

This website mentions many of the more famous transitional fossils, but these represent only a tiny fraction of what we know. We have good fossil evidence for a great many other transitions, some of which might be added here in the future.

Acknowledgements / contact

Tetrapods and Mixosaurus images copyright 2007 Nobu Tamura; released under the Creative Commons Attribution 2.5 License. Eocaecilia image copyright 2007 Nobu Tamura; released under the Creative Commons Attribution 3.0 License. Lycaenops image by Dmitry Bogdanov; released into the public domain. Whale images copyright 2006 Sharon Mooney; based on images from National Geographic, November 2001: "The Evolution of Whales" by Douglas H. Chadwick, Shawn Gould and Robert Clark; but re-illustrated for public access distribution by Sharon Mooney.

The author is a biology graduate. To contact me, send a message to any email address at this domain (except "info", which gets too much spam).

References

Articles behind a paywall are accessible for free from some universities and libraries. In any case, you can usually get at least an abstract.

Yates AM, Bonnan MF, Neveling J, Anusuya C, Blackbeard MG (2009) A new transitional sauropodomorph dinosaur from the Early Jurassic of South Africa and the evolution of sauropod feeding and quadrupedalism. Proceedings of the Royal Society B (online).

Zimmer C (1998) At the Water's Edge: Macroevolution and the Transformation of Life. New York: Free Press.